The present invention is directed to a valve for controlling pressure in a system which may either be a compressor, a vacuum pump or other device creating a differential pressure. The valve has a housing with an internal valve seat separating the housing into a first and second portion which have ports, a valve member is biased on the valve seat and has a rod for connecting it to a membrane mounted in a membrane housing so that pressure acting on two sides of the membrane, when reaching a determined differential will counteract a spring urging the valve member to a closed position.
U.S. Pat. No. 4,158,437, whose disclosure is incorporated by reference and which claims priority from German Application No. 24 59 485, discloses a thermostatic expansion valve for a refrigerator plant. The valve has a housing which is separated into two portions by a valve seat, each portion having a port. A valve member is urged into a closing position on the valve seat by a spring and has a valve rod connected to a membrane in a membrane housing. One side or surface of the membrane has a capillary tube inlet, and the other side or surface of the membrane is engaged by a bellows, which is in communication with the fluid pressure in the one portion of the housing while the remaining portion of that other surface of the membrane has pressure from the other portion of the valve housing acting thereon. Thus, when the pressure differential acting on the membrane reaches a certain value, the valve will open to allow passage from the first port to the second port. Such a valve is specifically intended to serve for employment as a thermostatic expansion valve in a refrigerating plant. As a consequence, it requires construction which is far too expensive for many application, for example, seals on the various valve stem members.